The present invention relates to membranes and, more particularly, to membranes which, under certain embodiments, serve to selectively control the diffusion of gases through the membrane. Additionally, the membrane not only selectively controls the diffusion of gases through the membrane, but also allows for the controlled diffusion of gases normally contained in the atmosphere.
Membranes, and more particularly, membranes useful for containing fluids, including liquids and/or gases, in a controlled manner, have been employed for years in a wide variety of products ranging from bladders useful in inflatable objects, including vehicle tires and sporting goods for example; to accumulators used on heavy machinery; to cushioning devices useful in footwear. Regardless of the intended use, membranes must generally be flexible, resistant to environmental degradation and exhibit excellent gas transmission controls. Often, however, materials which exhibit acceptable flexibility characteristics tend to have an unacceptably low level of resistance to gas permeation. In contrast, materials which exhibit an acceptable level of resistance to gas permeation tend to have an unacceptably low level of flexibility.
In an attempt to address the concerns of both flexibility and imperviousness to gases, U.S. Pat. No. 5,036,110 which issued Jun. 30, 1991, to Moreaux describes resilient membranes for fitting hydropneumatic accumulators. According to Moreaux ""110, the membrane disclosed consists of a film formed from a graft polymer which is the reaction product of an aromatic thermoplastic polyurethane with a copolymer of ethylene and vinyl alcohol, with this film being sandwiched between layers of thermoplastic polyurethane to form a laminate. While Moreaux ""110 attempts to address the concerns in the art relating to flexibility and imperviousness to gases, a perceived drawback of Moreaux is that the film described is not processable utilizing conventional techniques such as sheet extrusion, for example. Thus, the present invention is directed to membranes which are flexible, have good resistance to gas transmission, and under certain embodiments are processable into laminates utilizing conventional techniques such as sheet extrusion which are highly resistant to delamination.
While it should be understood by those skilled in the art upon review of the following specification and claims that the membranes of the present invention have a broad range of applications, including but not limited to bladders for inflatable objects such as footballs, basketballs, soccer balls, inner tubes; substantially rigid flotation devices such as boat hulls; flexible floatation devices such as tubes or rafts; as a component of medical equipment such as catheter balloons; fuel lines and fuel storage tanks; various cushioning devices such as those incorporated as part of an article of footwear or clothing; as part of an article of furniture such as chairs and seats, as part of a bicycle or saddle, as part of protective equipment including shin guards and helmets; as a supporting element for articles of furniture and, more particularly, lumbar supports; as part of a prosthetic or orthopedic device; as a portion of a vehicle tire and particularly, the outer layer of the tire, as well as being incorporated as part of certain recreation equipment such as components of wheels for in-line or roller skates, to name a few, still other applications are possible. For example, one highly desirable application for the membranes of the present invention include their use in forming accumulators which are operable under high pressure environments such as hydraulic accumulators as will be discussed in greater detail below.
For convenience, but without limitation, the membranes of the present invention will hereinafter generally be described in terms of either accumulators or in terms of still another highly desirable application, namely for cushioning devices used in footwear. In order to fully discuss the applicability of the membranes in terms of cushioning devices for footwear, a description of footwear in general is believed to be necessary.
Footwear, or more precisely, shoes generally include two major categories of components namely, a shoe upper and the sole. The general purpose of the shoe upper is to snugly and comfortably enclose the foot. Ideally, the shoe upper should be made from an attractive, highly durable, yet comfortable material or combination of materials. The sole, which also can be made from one or more durable materials, is particularly designed to provide traction and protect the wearer""s feet and body during use. The considerable forces generated during athletic activities require that the sole of an athletic shoe provide enhanced protection and shock absorption for the feet, ankles and legs of the wearer. For example, impacts which occur during running activities can generate forces of up to 2-3 times the body weight of an individual while certain other activities such as, for example, playing basketball have been known to generate forces of up to approximately 6-10 times an individual""s body weight. Accordingly, many shoes and, more particularly, many athletic shoes are now provided with some type of resilient, shock-absorbent material or shock-absorbent components to cushion the user during strenuous athletic activity. Such resilient, shock-absorbent materials or components have now commonly come to be referred to in the shoe manufacturing industry as the midsole.
It has therefore been a focus of the industry to seek midsole designs which achieve an effective impact response in which both adequate shock absorption and resiliency are appropriately taken into account. Such resilient, shock-absorbent materials or components could also be applied to the insole portion of the shoe, which is generally defined as the portion of the shoe upper directly underlining the plantar surface of the foot.
A particular focus in the footwear manufacturing industry has been to seek midsole or insert structure designs which are adapted to contain fluids, in either the liquid or gaseous state, or both. Examples of gas-filled structures which are utilized within the soles of shoes are shown in U.S. Pat. No. 900,867 entitled xe2x80x9cCushion for Footwearxe2x80x9d which issued Oct. 13, 1908, to Miller; U.S. Pat. No. 1,069,001 entitled xe2x80x9cCushioned Sole and Heel for Shoesxe2x80x9d which issued Jul. 29, 1913, to Guy; U.S. Pat. No. 1,304,915 entitled xe2x80x9cPneumatic Insolexe2x80x9d which issued May 27, 1919, to Spinney; U.S. Pat. No. 1,514,468 entitled xe2x80x9cArch Cushionxe2x80x9d which issued Nov. 4, 1924, to Schopf; U.S. Pat. No. 2,080,469 entitled xe2x80x9cPneumatic Foot Supportxe2x80x9d which issued May 18, 1937, to Gilbert; U.S. Pat. No. 2,645,865 entitled xe2x80x9cCushioning Insole for Shoesxe2x80x9d which issued Jul. 21, 1953, to Towne; U.S. Pat. No. 2,677,906 entitled xe2x80x9cCushioned Inner Sole for Shoes and Method of Making the Samexe2x80x9d which issued May 11, 1954, to Reed; U.S. Pat. No. 4,183,156 entitled xe2x80x9cInsole Construction for Articles of Footwearxe2x80x9d which issued Jan. 15, 1980, to Rudy; U.S. Pat. No. 4,219,945 entitled xe2x80x9cFootwearxe2x80x9d which issued Sep. 2, 1980, also to Rudy; U.S. Pat. No. 4,722,131 entitled xe2x80x9cAir Cushion Shoe Solexe2x80x9d which issued Feb. 2, 1988, to Huang; and U.S. Pat. No. 4,864,738 entitled xe2x80x9cSole Construction for Footwearxe2x80x9d which issued Sep. 12, 1989, to Horovitz. As will be recognized by those skilled in the art, such gas filled structures often referred to in the shoe manufacturing industry as xe2x80x9cbladdersxe2x80x9d typically fall into two broad categories, namely (1) xe2x80x9cpermanentlyxe2x80x9d inflated systems such as those disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 and (2) pump and valve adjustable systems as exemplified by U.S. Pat. No. 4,722,131. By way of further example, athletic shoes of the type disclosed in U.S. Pat. No. 4,182,156 which include xe2x80x9cpermanentlyxe2x80x9d inflated bladders have been successfully sold under the trade mark xe2x80x9cAir-Solexe2x80x9d and other trademarks by Nike, Inc. of Beaverton, Oreg. To date, millions of pairs of athletic shoes of this type have been sold in the United States and throughout the world.
The permanently inflated bladders have historically been constructed under methods using a flexible thermoplastic material which is inflated with a large molecule, low solubility coefficient gas otherwise referred to in the industry as a xe2x80x9csuper gas.xe2x80x9d By way of example, U.S. Pat. No. 4,340,626 entitled xe2x80x9cDiffusion Pumping Apparatus Self-Inflating Devicexe2x80x9d which issued Jul. 20, 1982, to Rudy, which is expressly incorporated herein by reference, discloses selectively permeable sheets of film which are formed into a bladder and thereafter inflated with a gas or mixture of gases to a prescribed pressure which preferably is above atmospheric pressure. The gas or gases utilized ideally have a relatively low diffusion rate through the selectively permeable bladder to the exterior environment while gases such as nitrogen, oxygen and argon which are contained in the atmosphere and have a relatively high diffusion rate are able to penetrate the bladder. This produces an increase in the total pressure within the bladder, by the addition of the partial pressures of the nitrogen, oxygen and argon from the atmosphere to the partial pressures of the gas or gases contained initially injected into the bladder upon inflation. This concept of a relative one-way addition of gases to enhance the total pressure of the bladder is now known as xe2x80x9cdiffusion pumping.xe2x80x9d
With regard to the systems utilized within the footwear manufacturing industry prior to and shortly after the introduction of the Air-Sole(trademark) athletic shoes, many of the midsole bladders consisted of a single layer gas barrier type films made from polyvinylidene chloride based materials such as Saran(copyright) (which is a registered trademark of the Dow Chemical Co.) and which by their nature are rigid plastics, having relatively poor flex fatigue, heat sealability and elasticity.
Still further, bladder films made under techniques such as laminations and coatings which involve one or more barrier materials in combination with a flexible bladder material (such as various thermoplastics) can potentially present a wide variety of problems to solve. Such difficulties with composite constructions include layer separation, peeling, gas diffusion or capillary action at weld interfaces, low elongation which leads to wrinkling of the inflated product, cloudy appearing finished bladders, reduced puncture resistance and tear strength, resistance to formation via blow-molding and/or heat-sealing and RF welding, high cost processing, and difficulty with foam encapsulation and adhesive bonding, among others.
Yet another issue with previously known multi-layer bladders is the use of tie-layers or adhesives in preparing laminates. The use of such tie layers or adhesives generally prevent regrinding and recycling of any waste materials created during product formation back into an usable product, and thus, also contribute to high cost of manufacturing and relative waste. These and other perceived short comings of the prior art are described in more extensive detail in U.S. Pat. Nos. 4,340,626; 4,936,029 and 5,042,176, all of which are hereby expressly incorporated by reference.
Previously known multi-layer bladders which specifically eliminate adhesive tie layers have been known to separate or de-laminate especially along seams and edges. Thus, it has been a relatively recent focus of the industry to develop laminated bladders which reduce or eliminate the occurrence of delamination ideally without the use of a xe2x80x9ctie layer.xe2x80x9d In this regard, the cushioning devices disclosed in co-pending U.S. application Ser. Nos. 08/299,286 and 08/299,287 eliminate adhesive tie layers by providing membranes including a first layer of thermoplastic urethane and a second layer including a barrier material such as a copolymer of ethylene and vinyl alcohol wherein hydrogen bonding occurs over a segment of the membranes between the first and second layers. While the membranes disclosed in U.S. patent application Ser. No. 08/299,287 and the laminated flexible membranes of U.S. patent application Ser. No. 08/299,286 are believed to offer a significant improvement in the art, still further improvements are offered according to the teachings of the present invention.
With the extensive commercial success of the products such as the Air-Sole(trademark) shoes, consumers have been able to enjoy products with a long service life, superior shock absorbency and resiliency, reasonable cost, and inflation stability, without having to resort to pumps and valves. Thus, in light of the significant commercial acceptance and success that has been achieved through the use of long life inflated gas filled bladders, it is highly desirable to develop advancements relating to such products. One goal then is to provide flexible, xe2x80x9cpermanentlyxe2x80x9d inflated, gas-filled shoe cushioning components which meet, and hopefully exceed, performance achieved by such products as the Air-Sole(trademark) athletic shoes offered by Nike, Inc.
An accepted method of measuring the relative permeance, permeability and diffusion of different film materials is set forth in the procedure designated as ASTM D-1434-82-V. According to ASTM D-1434-82-V, permeance, permeability and diffusion are measured by the following formulas:             Permeance      _        ⁢          xe2x80x83                          (                  quantity          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          gas                )                              (          area          )                xc3x97                  (          time          )                xc3x97                  (                      press            .                          xe2x80x83                        ⁢            diff            .                    )                      =                                        Permeance                                                                              (                GTR                )                            /                              (                                  press                  .                                      xe2x80x83                                    ⁢                  diff                  .                                )                                                        =                        cc          .                                      (                          sq              .                              xe2x80x83                            ⁢              m                        )                    ⁢                      (                          24              ⁢                              xe2x80x83                            ⁢              hr                        )                    ⁢                      (            Pa            )                                          Permeability      _        ⁢                  xe2x80x83            ⁢              xe2x80x83                                          (                      quantity            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            gas                    )                xc3x97                  (                      film            ⁢                          xe2x80x83                        ⁢            thick                    )                                      (          area          )                xc3x97                  (          time          )                xc3x97                  (                      press            .                          xe2x80x83                        ⁢            diff            .                    )                      =                                        Permeability                                                                              (                GTR                )                            xc3x97                                                (                                      film                    ⁢                                          xe2x80x83                                        ⁢                    thick                                    )                                /                                  (                                      press                    .                                          xe2x80x83                                        ⁢                    diff                    .                                    )                                                                        =                                    (            cc            )                    ⁢                      (            mil            )                                                (                          sq              .                              xe2x80x83                            ⁢              m                        )                    ⁢                      (                          24              ⁢                              xe2x80x83                            ⁢              hr                        )                    ⁢                      (            Pa            )                                          Diffusion      _        ⁢                  xe2x80x83            ⁢              xe2x80x83                                (                  quantity          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          gas                )                              (          area          )                xc3x97                  (          time          )                      =                                                      Gas              ⁢                              xe2x80x83                            ⁢              Transmission              ⁢                              xe2x80x83                            ⁢              Rate                                                                          (              GTR              )                                          =              cc                              (                          sq              .                              xe2x80x83                            ⁢              m                        )                    ⁢                      (                          24              ⁢                              xe2x80x83                            ⁢              hr                        )                              
By utilizing the above listed formulas, the gas transmission rate in combination with a constant pressure differential and the film""s thickness, can be utilized to define the movement of gas under specific conditions. In this regard, the preferred gas transmission rate (GTR) for a membrane having an average thickness of approximately 20.0 mils such as those useful for forming a cushioning device used as a shoe component which seeks to meet the rigorous demands of fatigue resistance imposed by heavy and repeated impacts will preferably have a gas transmission rate (GTR) of 15.0 or less for nitrogen gas according to ASTM D-1434-82-V. More preferably, the membranes will have a GTR of less than about 2.0 at an average thickness of 20 mils.
It is, therefore, one object of the present invention to provide membranes including both single layer and multi-layer constructions which offer enhanced flexibility, durability and resistance to the undesired transmission of fluids therethrough.
It is another object of the present invention to provide membranes which can be inflated with a gas such as nitrogen wherein the membrane provides for a gas transmission rate value of 15.0 or less, based on a 20 mils average thickness.
It is still another object of the present invention to provide membranes, particularly those employed as cushioning devices, having a relatively high degree of transparency.
It is another object of the present invention to provide monolayer membranes which are readily processable into various products.
It is yet another object of the present invention to provide monolayer membranes and, under certain applications, multi-layer membranes which are reprocessable and repairable.
It is yet another object of the present invention to provide membranes which can be formed into laminated objects such as cushioning devices or accumulators, among others, which better resist delamination and also may not require a tie layer between the layers.
It is a further object of the present invention to provide membranes which are formable utilizing various techniques including, but not limited to, blow-molding, tubing, sheet extrusion, vacuum-forming, heat-sealing, casting, liquid casting, low pressure casting, spin casting, reaction injection molding and RF welding.
Still another object of the present invention is to provide membranes which prevent gas from escaping along interfaces between the layers in laminated embodiments and particularly along seems via capillary action.
It is yet another object of the present invention to provide a membrane which allows for footwear processing such as encapsulation of a membrane within a formable material.
While the aforementioned objects provide guidance as to possible applications and advantages for the membranes of the present invention, it should be recognized by those skilled in the art that the recited objects are not intended to be exhaustive or limiting.
To achieve the foregoing objects, the present invention provides membranes which preferably have one or more of the following: (1) a desirable level of flexibility (or rigidity); (2) a desirable level of resistance to degradation caused by moisture; (3) an acceptable level of imperviousness to fluids which can be in the form of gases, liquids or both depending mainly on the intended use of the product; and (4) resistance to delamination when employed in a multi-layer structure. Regardless of the membrane embodiment, each membrane in accordance with the teachings of the present invention includes a layer comprised of a polyester polyol based polyurethane. The aforementioned layer may also include at least one barrier material selected from the group consisting of co-polymers of ethylene and vinyl alcohol, polyvinylidene chloride, co-polymers of acrylonitrile and methyl acrylate, polyethylene terephthalate, aliphatic and aromatic polyamides, crystalline polymers and polyurethane engineering thermoplastics blended with the polyurethane prior to forming the membranes.
The polyester polyol based urethanes employed, if not commercially available, are preferably formed as the reaction product of (a) one or more carboxylic acids having six or less carbon atoms with one or more diols having six or less carbon atoms; (b) at least one isocyanate and/or diisocyanate; and (c) optionally, but preferably, one or more extenders. The polyester polyol may also include a relatively small amount of one or more polyfunctional materials such as triols which are included as part of the reaction product. In addition to the foregoing, the polyester polyol based urethanes may optionally employ one or more of the following: (d) hydrolytic stabilizers; (e) plasticizers; (f) fillers; (g) flame retardants; and (h) processing aids. The resulting polyester polyols formed as a result of the reaction product of the one or more carboxylic acids with one or more diols preferably have repeating units containing eight carbon atoms or less.
The term xe2x80x9ccarboxylic acidxe2x80x9d as used herein, and unless otherwise indicated, preferably means a carboxylic acid, and more preferably a dicarboxylic acid, having no more than six carbon atoms when reacted with a diol, wherein the repeating units of the polyester polyol formed by the aforesaid reaction has no more than eight carbon atoms.
The term xe2x80x9cdiolxe2x80x9d as used herein, and unless otherwise indicated, to preferably mean diols having no more than six carbon atoms when reacted with a carboxylic acid, wherein the repeating units of the polyester polyol formed by the aforesaid reaction has no more than eight carbon atoms.
The term xe2x80x9cpolyester polyolxe2x80x9d as used herein is intended to preferably mean polymeric polyester polyols having a molecular weight (determined by the ASTM D-4274 method) falling in the range of about 300 to about 4,000; more preferably from about 400 to about 2,000; and still more preferably between about 500 to about 1,500.
The term xe2x80x9cthermoplasticxe2x80x9d as used herein is generally intended to mean that the material is capable of being softened by heating and hardened by cooling through a characteristic temperature range, and as such in the softened state can be shaped into various articles under various techniques.
The term xe2x80x9cthermosetxe2x80x9d as used herein is generally intended to mean a polymeric material that will not flow upon the application of heat and pressure after it is substantially reacted.
The term xe2x80x9cextenderxe2x80x9d or xe2x80x9cdifunctional extenderxe2x80x9d is used preferably in the commonly accepted sense to one skilled in the art and includes glycols, diamines, amino alcohols and the like. Preferably, any such extender or difunctional extender employed in accordance with the teachings of the present invention will have a molecular weight generally failing in the range of from about 60 to about 400.
The term xe2x80x9csoft segmentxe2x80x9d as used herein is generally intended to mean the component of the formulation exhibiting a molecular weight from approximately 300-4000 that contains approximately two or more active hydrogen groups per molecule prior to reaction that provides the elastomeric character of the resulting polymers.
Preferably, the membranes described herein may be useful as components for footwear. In such applications, the membranes preferably are capable of containing a captive gas for a relatively long period of time. In a highly preferred embodiment, for example, the membrane should not lose more than about 20% of the initial inflated gas pressure over a period of approximately two years. In other words, products inflated initially to a steady state pressure of between 20.0 to 22.0 psi should retain pressure in the range of about 16.0 to 18.0 psi for at least about two years.
Additionally, the materials utilized for products such as components of athletic shoes should be flexible, relatively soft and compliant and should be highly resistant to fatigue and be capable of being welded to form effective seals typically achieved by RF welding or heat sealing. The material should also have the ability to withstand high cycle loads without failure, especially when the material utilized has a thickness of between about 5 mils to about 200 mils.
Another preferred characteristic of the membrane is the ability to be processable into various shapes by techniques used in high volume production. Among these techniques known in the art are extrusion, blow molding, injection molding, vacuum molding, rotary molding, transfer molding, pressure forming, heat-sealing, casting, low pressure casting, spin casting, reaction injection molding and RF welding, among others.
As discussed above, a preferred characteristic of the membranes, whether monolayer or multi-layer in construction, is their ability under embodiments to be formed into products which are inflated (such as cushioning devices for footwear) and which control diffusion of mobile gases through the membrane. By the present invention, not only are super gases usable as captive gases, but nitrogen gas and air, among others, may also be used as captive gases due to the performance of the materials.
Another feature of the monolayer membranes of the present invention is elimination of many of the processing concerns presented by multi-layer embodiments. Monolayer membranes can generally be processed without requiring special mechanical adapters for processing equipment and other process controls. Further, products formed from monolayer embodiments are not subject to delamination and can, at least in the case of thermoplastics, be recycled and reground for subsequent inclusion in a variety of products.
With regard to multiple layer embodiments, a further feature of the present invention is the enhanced bonding which can occur between contiguous layers, thus, potentially eliminating the need for adhesive tie layers. This so-called enhanced bonding is generally accomplished by bringing the first and second layers together into intimate contact using conventional techniques wherein the materials of both layers have available functional groups with hydrogen atoms that can participate in hydrogen bonding such as hydrogen atoms in hydroxyl groups or hydrogen atoms attached to nitrogen atoms in urethane groups and various receptor groups such as oxygen atoms in hydroxyl groups, carboxyl oxygens in urethane groups and ester groups, and chlorine atoms in PVDC, for example. Such laminated membranes are characterized in that hydrogen bonding is believed to occur between the first and second layers. For example, the above described hydrogen bonding will theoretically occur where the first layer comprises a polyester polyol based urethane and the second layer includes a barrier material such as one selected from the group consisting of co-polymers of ethylene and vinyl alcohol, polyvinylidene chloride, co-polymers of acrylonitrile and methyl acrylate, polyethylene terephthalate, aliphatic and aromatic polyamides, crystalline polymers and polyurethane engineering thermoplastics. In addition to the occurrence of hydrogen bonding, it is theorized that there will also generally be a certain amount of covalent bonding between the first and second layers if, for example, there are polyurethanes in adjacent layers or if one of the layers includes polyurethane and the adjacent layer includes a barrier material such as copolymers of ethylene and vinyl alcohol.
This invention has many other advantages which will be more apparent from consideration of the various forms and embodiments of the present invention. Again, while the embodiments shown in the accompanying drawings which form a part of the present specification are illustrative of embodiments employing the membranes of the present invention, it should be clear that the membranes have extensive application possibilities. Various exemplary embodiments will now be described in greater detail for the purpose of illustrating the general principles of the invention, without considering the following detailed description in the limiting sense.